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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util.stream;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.Objects;
import java.util.OptionalDouble;
import java.util.OptionalInt;
import java.util.PrimitiveIterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.IntBinaryOperator;
import java.util.function.IntConsumer;
import java.util.function.IntFunction;
import java.util.function.IntPredicate;
import java.util.function.IntSupplier;
import java.util.function.IntToDoubleFunction;
import java.util.function.IntToLongFunction;
import java.util.function.IntUnaryOperator;
import java.util.function.ObjIntConsumer;
import java.util.function.Supplier;

/**
 * A sequence of primitive int-valued elements supporting sequential and parallel
 * aggregate operations.  This is the {@code int} primitive specialization of
 * {@link Stream}.
 *
 * <p>The following example illustrates an aggregate operation using
 * {@link Stream} and {@link IntStream}, computing the sum of the weights of the
 * red widgets:
 *
 * <pre>{@code
 *     int sum = widgets.stream()
 *                      .filter(w -> w.getColor() == RED)
 *                      .mapToInt(w -> w.getWeight())
 *                      .sum();
 * }</pre>
 *
 * See the class documentation for {@link Stream} and the package documentation
 * for <a href="package-summary.html">java.util.stream</a> for additional
 * specification of streams, stream operations, stream pipelines, and
 * parallelism.
 *
 * @see Stream
 * @see <a href="package-summary.html">java.util.stream</a>
 * @since 1.8
 */
public interface IntStream extends BaseStream<Integer, IntStream> {

  /**
   * Returns a stream consisting of the elements of this stream that match
   * the given predicate.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> predicate to apply to each element to
   * determine if it should be included
   * @return the new stream
   */
  IntStream filter(IntPredicate predicate);

  /**
   * Returns a stream consisting of the results of applying the given
   * function to the elements of this stream.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function to apply to each element
   * @return the new stream
   */
  IntStream map(IntUnaryOperator mapper);

  /**
   * Returns an object-valued {@code Stream} consisting of the results of
   * applying the given function to the elements of this stream.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">
   * intermediate operation</a>.
   *
   * @param <U> the element type of the new stream
   * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function to apply to each element
   * @return the new stream
   */
  <U> Stream<U> mapToObj(IntFunction<? extends U> mapper);

  /**
   * Returns a {@code LongStream} consisting of the results of applying the
   * given function to the elements of this stream.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function to apply to each element
   * @return the new stream
   */
  LongStream mapToLong(IntToLongFunction mapper);

  /**
   * Returns a {@code DoubleStream} consisting of the results of applying the
   * given function to the elements of this stream.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function to apply to each element
   * @return the new stream
   */
  DoubleStream mapToDouble(IntToDoubleFunction mapper);

  /**
   * Returns a stream consisting of the results of replacing each element of
   * this stream with the contents of a mapped stream produced by applying
   * the provided mapping function to each element.  Each mapped stream is
   * {@link java.util.stream.BaseStream#close() closed} after its contents
   * have been placed into this stream.  (If a mapped stream is {@code null}
   * an empty stream is used, instead.)
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function to apply to each element which
   * produces an {@code IntStream} of new values
   * @return the new stream
   * @see Stream#flatMap(Function)
   */
  IntStream flatMap(IntFunction<? extends IntStream> mapper);

  /**
   * Returns a stream consisting of the distinct elements of this stream.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">stateful
   * intermediate operation</a>.
   *
   * @return the new stream
   */
  IntStream distinct();

  /**
   * Returns a stream consisting of the elements of this stream in sorted
   * order.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">stateful
   * intermediate operation</a>.
   *
   * @return the new stream
   */
  IntStream sorted();

  /**
   * Returns a stream consisting of the elements of this stream, additionally
   * performing the provided action on each element as elements are consumed
   * from the resulting stream.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * <p>For parallel stream pipelines, the action may be called at
   * whatever time and in whatever thread the element is made available by the
   * upstream operation.  If the action modifies shared state,
   * it is responsible for providing the required synchronization.
   *
   * @param action a <a href="package-summary.html#NonInterference"> non-interfering</a> action to
   * perform on the elements as they are consumed from the stream
   * @return the new stream
   * @apiNote This method exists mainly to support debugging, where you want to see the elements as
   * they flow past a certain point in a pipeline:
   * <pre>{@code
   *     IntStream.of(1, 2, 3, 4)
   *         .filter(e -> e > 2)
   *         .peek(e -> System.out.println("Filtered value: " + e))
   *         .map(e -> e * e)
   *         .peek(e -> System.out.println("Mapped value: " + e))
   *         .sum();
   * }</pre>
   */
  IntStream peek(IntConsumer action);

  /**
   * Returns a stream consisting of the elements of this stream, truncated
   * to be no longer than {@code maxSize} in length.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * stateful intermediate operation</a>.
   *
   * @param maxSize the number of elements the stream should be limited to
   * @return the new stream
   * @throws IllegalArgumentException if {@code maxSize} is negative
   * @apiNote While {@code limit()} is generally a cheap operation on sequential stream pipelines,
   * it can be quite expensive on ordered parallel pipelines, especially for large values of {@code
   * maxSize}, since {@code limit(n)} is constrained to return not just any <em>n</em> elements, but
   * the <em>first n</em> elements in the encounter order.  Using an unordered stream source (such
   * as {@link #generate(IntSupplier)}) or removing the ordering constraint with {@link
   * #unordered()} may result in significant speedups of {@code limit()} in parallel pipelines, if
   * the semantics of your situation permit.  If consistency with encounter order is required, and
   * you are experiencing poor performance or memory utilization with {@code limit()} in parallel
   * pipelines, switching to sequential execution with {@link #sequential()} may improve
   * performance.
   */
  IntStream limit(long maxSize);

  /**
   * Returns a stream consisting of the remaining elements of this stream
   * after discarding the first {@code n} elements of the stream.
   * If this stream contains fewer than {@code n} elements then an
   * empty stream will be returned.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">stateful
   * intermediate operation</a>.
   *
   * @param n the number of leading elements to skip
   * @return the new stream
   * @throws IllegalArgumentException if {@code n} is negative
   * @apiNote While {@code skip()} is generally a cheap operation on sequential stream pipelines, it
   * can be quite expensive on ordered parallel pipelines, especially for large values of {@code n},
   * since {@code skip(n)} is constrained to skip not just any <em>n</em> elements, but the
   * <em>first n</em> elements in the encounter order.  Using an unordered stream source (such as
   * {@link #generate(IntSupplier)}) or removing the ordering constraint with {@link #unordered()}
   * may result in significant speedups of {@code skip()} in parallel pipelines, if the semantics of
   * your situation permit.  If consistency with encounter order is required, and you are
   * experiencing poor performance or memory utilization with {@code skip()} in parallel pipelines,
   * switching to sequential execution with {@link #sequential()} may improve performance.
   */
  IntStream skip(long n);

  /**
   * Performs an action for each element of this stream.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * <p>For parallel stream pipelines, this operation does <em>not</em>
   * guarantee to respect the encounter order of the stream, as doing so
   * would sacrifice the benefit of parallelism.  For any given element, the
   * action may be performed at whatever time and in whatever thread the
   * library chooses.  If the action accesses shared state, it is
   * responsible for providing the required synchronization.
   *
   * @param action a <a href="package-summary.html#NonInterference"> non-interfering</a> action to
   * perform on the elements
   */
  void forEach(IntConsumer action);

  /**
   * Performs an action for each element of this stream, guaranteeing that
   * each element is processed in encounter order for streams that have a
   * defined encounter order.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @param action a <a href="package-summary.html#NonInterference"> non-interfering</a> action to
   * perform on the elements
   * @see #forEach(IntConsumer)
   */
  void forEachOrdered(IntConsumer action);

  /**
   * Returns an array containing the elements of this stream.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @return an array containing the elements of this stream
   */
  int[] toArray();

  /**
   * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
   * elements of this stream, using the provided identity value and an
   * <a href="package-summary.html#Associativity">associative</a>
   * accumulation function, and returns the reduced value.  This is equivalent
   * to:
   * <pre>{@code
   *     int result = identity;
   *     for (int element : this stream)
   *         result = accumulator.applyAsInt(result, element)
   *     return result;
   * }</pre>
   *
   * but is not constrained to execute sequentially.
   *
   * <p>The {@code identity} value must be an identity for the accumulator
   * function. This means that for all {@code x},
   * {@code accumulator.apply(identity, x)} is equal to {@code x}.
   * The {@code accumulator} function must be an
   * <a href="package-summary.html#Associativity">associative</a> function.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @param identity the identity value for the accumulating function
   * @param op an <a href="package-summary.html#Associativity">associative</a>, <a
   * href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function for combining two values
   * @return the result of the reduction
   * @apiNote Sum, min, max, and average are all special cases of reduction. Summing a stream of
   * numbers can be expressed as:
   *
   * <pre>{@code
   *     int sum = integers.reduce(0, (a, b) -> a+b);
   * }</pre>
   *
   * or more compactly:
   *
   * <pre>{@code
   *     int sum = integers.reduce(0, Integer::sum);
   * }</pre>
   *
   * <p>While this may seem a more roundabout way to perform an aggregation compared to simply
   * mutating a running total in a loop, reduction operations parallelize more gracefully, without
   * needing additional synchronization and with greatly reduced risk of data races.
   * @see #sum()
   * @see #min()
   * @see #max()
   * @see #average()
   */
  int reduce(int identity, IntBinaryOperator op);

  /**
   * Performs a <a href="package-summary.html#Reduction">reduction</a> on the
   * elements of this stream, using an
   * <a href="package-summary.html#Associativity">associative</a> accumulation
   * function, and returns an {@code OptionalInt} describing the reduced value,
   * if any. This is equivalent to:
   * <pre>{@code
   *     boolean foundAny = false;
   *     int result = null;
   *     for (int element : this stream) {
   *         if (!foundAny) {
   *             foundAny = true;
   *             result = element;
   *         }
   *         else
   *             result = accumulator.applyAsInt(result, element);
   *     }
   *     return foundAny ? OptionalInt.of(result) : OptionalInt.empty();
   * }</pre>
   *
   * but is not constrained to execute sequentially.
   *
   * <p>The {@code accumulator} function must be an
   * <a href="package-summary.html#Associativity">associative</a> function.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @param op an <a href="package-summary.html#Associativity">associative</a>, <a
   * href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function for combining two values
   * @return the result of the reduction
   * @see #reduce(int, IntBinaryOperator)
   */
  OptionalInt reduce(IntBinaryOperator op);

  /**
   * Performs a <a href="package-summary.html#MutableReduction">mutable
   * reduction</a> operation on the elements of this stream.  A mutable
   * reduction is one in which the reduced value is a mutable result container,
   * such as an {@code ArrayList}, and elements are incorporated by updating
   * the state of the result rather than by replacing the result.  This
   * produces a result equivalent to:
   * <pre>{@code
   *     R result = supplier.get();
   *     for (int element : this stream)
   *         accumulator.accept(result, element);
   *     return result;
   * }</pre>
   *
   * <p>Like {@link #reduce(int, IntBinaryOperator)}, {@code collect} operations
   * can be parallelized without requiring additional synchronization.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @param <R> type of the result
   * @param supplier a function that creates a new result container. For a parallel execution, this
   * function may be called multiple times and must return a fresh value each time.
   * @param accumulator an <a href="package-summary.html#Associativity">associative</a>, <a
   * href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function for incorporating an
   * additional element into a result
   * @param combiner an <a href="package-summary.html#Associativity">associative</a>, <a
   * href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> function for combining two values,
   * which must be compatible with the accumulator function
   * @return the result of the reduction
   * @see Stream#collect(Supplier, BiConsumer, BiConsumer)
   */
  <R> R collect(Supplier<R> supplier,
      ObjIntConsumer<R> accumulator,
      BiConsumer<R, R> combiner);

  /**
   * Returns the sum of elements in this stream.  This is a special case
   * of a <a href="package-summary.html#Reduction">reduction</a>
   * and is equivalent to:
   * <pre>{@code
   *     return reduce(0, Integer::sum);
   * }</pre>
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @return the sum of elements in this stream
   */
  int sum();

  /**
   * Returns an {@code OptionalInt} describing the minimum element of this
   * stream, or an empty optional if this stream is empty.  This is a special
   * case of a <a href="package-summary.html#Reduction">reduction</a>
   * and is equivalent to:
   * <pre>{@code
   *     return reduce(Integer::min);
   * }</pre>
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
   *
   * @return an {@code OptionalInt} containing the minimum element of this stream, or an empty
   * {@code OptionalInt} if the stream is empty
   */
  OptionalInt min();

  /**
   * Returns an {@code OptionalInt} describing the maximum element of this
   * stream, or an empty optional if this stream is empty.  This is a special
   * case of a <a href="package-summary.html#Reduction">reduction</a>
   * and is equivalent to:
   * <pre>{@code
   *     return reduce(Integer::max);
   * }</pre>
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @return an {@code OptionalInt} containing the maximum element of this stream, or an empty
   * {@code OptionalInt} if the stream is empty
   */
  OptionalInt max();

  /**
   * Returns the count of elements in this stream.  This is a special case of
   * a <a href="package-summary.html#Reduction">reduction</a> and is
   * equivalent to:
   * <pre>{@code
   *     return mapToLong(e -> 1L).sum();
   * }</pre>
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
   *
   * @return the count of elements in this stream
   */
  long count();

  /**
   * Returns an {@code OptionalDouble} describing the arithmetic mean of elements of
   * this stream, or an empty optional if this stream is empty.  This is a
   * special case of a
   * <a href="package-summary.html#Reduction">reduction</a>.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @return an {@code OptionalDouble} containing the average element of this stream, or an empty
   * optional if the stream is empty
   */
  OptionalDouble average();

  /**
   * Returns an {@code IntSummaryStatistics} describing various
   * summary data about the elements of this stream.  This is a special
   * case of a <a href="package-summary.html#Reduction">reduction</a>.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">terminal
   * operation</a>.
   *
   * @return an {@code IntSummaryStatistics} describing various summary data about the elements of
   * this stream
   */
  IntSummaryStatistics summaryStatistics();

  /**
   * Returns whether any elements of this stream match the provided
   * predicate.  May not evaluate the predicate on all elements if not
   * necessary for determining the result.  If the stream is empty then
   * {@code false} is returned and the predicate is not evaluated.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * terminal operation</a>.
   *
   * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> predicate to apply to elements of this
   * stream
   * @return {@code true} if any elements of the stream match the provided predicate, otherwise
   * {@code false}
   * @apiNote This method evaluates the <em>existential quantification</em> of the predicate over
   * the elements of the stream (for some x P(x)).
   */
  boolean anyMatch(IntPredicate predicate);

  /**
   * Returns whether all elements of this stream match the provided predicate.
   * May not evaluate the predicate on all elements if not necessary for
   * determining the result.  If the stream is empty then {@code true} is
   * returned and the predicate is not evaluated.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * terminal operation</a>.
   *
   * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> predicate to apply to elements of this
   * stream
   * @return {@code true} if either all elements of the stream match the provided predicate or the
   * stream is empty, otherwise {@code false}
   * @apiNote This method evaluates the <em>universal quantification</em> of the predicate over the
   * elements of the stream (for all x P(x)).  If the stream is empty, the quantification is said to
   * be <em>vacuously satisfied</em> and is always {@code true} (regardless of P(x)).
   */
  boolean allMatch(IntPredicate predicate);

  /**
   * Returns whether no elements of this stream match the provided predicate.
   * May not evaluate the predicate on all elements if not necessary for
   * determining the result.  If the stream is empty then {@code true} is
   * returned and the predicate is not evaluated.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * terminal operation</a>.
   *
   * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>, <a
   * href="package-summary.html#Statelessness">stateless</a> predicate to apply to elements of this
   * stream
   * @return {@code true} if either no elements of the stream match the provided predicate or the
   * stream is empty, otherwise {@code false}
   * @apiNote This method evaluates the <em>universal quantification</em> of the negated predicate
   * over the elements of the stream (for all x ~P(x)).  If the stream is empty, the quantification
   * is said to be vacuously satisfied and is always {@code true}, regardless of P(x).
   */
  boolean noneMatch(IntPredicate predicate);

  /**
   * Returns an {@link OptionalInt} describing the first element of this
   * stream, or an empty {@code OptionalInt} if the stream is empty.  If the
   * stream has no encounter order, then any element may be returned.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * terminal operation</a>.
   *
   * @return an {@code OptionalInt} describing the first element of this stream, or an empty {@code
   * OptionalInt} if the stream is empty
   */
  OptionalInt findFirst();

  /**
   * Returns an {@link OptionalInt} describing some element of the stream, or
   * an empty {@code OptionalInt} if the stream is empty.
   *
   * <p>This is a <a href="package-summary.html#StreamOps">short-circuiting
   * terminal operation</a>.
   *
   * <p>The behavior of this operation is explicitly nondeterministic; it is
   * free to select any element in the stream.  This is to allow for maximal
   * performance in parallel operations; the cost is that multiple invocations
   * on the same source may not return the same result.  (If a stable result
   * is desired, use {@link #findFirst()} instead.)
   *
   * @return an {@code OptionalInt} describing some element of this stream, or an empty {@code
   * OptionalInt} if the stream is empty
   * @see #findFirst()
   */
  OptionalInt findAny();

  /**
   * Returns a {@code LongStream} consisting of the elements of this stream,
   * converted to {@code long}.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @return a {@code LongStream} consisting of the elements of this stream, converted to {@code
   * long}
   */
  LongStream asLongStream();

  /**
   * Returns a {@code DoubleStream} consisting of the elements of this stream,
   * converted to {@code double}.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @return a {@code DoubleStream} consisting of the elements of this stream, converted to {@code
   * double}
   */
  DoubleStream asDoubleStream();

  /**
   * Returns a {@code Stream} consisting of the elements of this stream,
   * each boxed to an {@code Integer}.
   *
   * <p>This is an <a href="package-summary.html#StreamOps">intermediate
   * operation</a>.
   *
   * @return a {@code Stream} consistent of the elements of this stream, each boxed to an {@code
   * Integer}
   */
  Stream<Integer> boxed();

  @Override
  IntStream sequential();

  @Override
  IntStream parallel();

  @Override
  PrimitiveIterator.OfInt iterator();

  @Override
  Spliterator.OfInt spliterator();

  // Static factories

  /**
   * Returns a builder for an {@code IntStream}.
   *
   * @return a stream builder
   */
  public static Builder builder() {
    return new Streams.IntStreamBuilderImpl();
  }

  /**
   * Returns an empty sequential {@code IntStream}.
   *
   * @return an empty sequential stream
   */
  public static IntStream empty() {
    return StreamSupport.intStream(Spliterators.emptyIntSpliterator(), false);
  }

  /**
   * Returns a sequential {@code IntStream} containing a single element.
   *
   * @param t the single element
   * @return a singleton sequential stream
   */
  public static IntStream of(int t) {
    return StreamSupport.intStream(new Streams.IntStreamBuilderImpl(t), false);
  }

  /**
   * Returns a sequential ordered stream whose elements are the specified values.
   *
   * @param values the elements of the new stream
   * @return the new stream
   */
  public static IntStream of(int... values) {
    return Arrays.stream(values);
  }

  /**
   * Returns an infinite sequential ordered {@code IntStream} produced by iterative
   * application of a function {@code f} to an initial element {@code seed},
   * producing a {@code Stream} consisting of {@code seed}, {@code f(seed)},
   * {@code f(f(seed))}, etc.
   *
   * <p>The first element (position {@code 0}) in the {@code IntStream} will be
   * the provided {@code seed}.  For {@code n > 0}, the element at position
   * {@code n}, will be the result of applying the function {@code f} to the
   * element at position {@code n - 1}.
   *
   * @param seed the initial element
   * @param f a function to be applied to to the previous element to produce a new element
   * @return A new sequential {@code IntStream}
   */
  public static IntStream iterate(final int seed, final IntUnaryOperator f) {
    Objects.requireNonNull(f);
    final PrimitiveIterator.OfInt iterator = new PrimitiveIterator.OfInt() {
      int t = seed;

      @Override
      public boolean hasNext() {
        return true;
      }

      @Override
      public int nextInt() {
        int v = t;
        t = f.applyAsInt(t);
        return v;
      }
    };
    return StreamSupport.intStream(Spliterators.spliteratorUnknownSize(
        iterator,
        Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.NONNULL), false);
  }

  /**
   * Returns an infinite sequential unordered stream where each element is
   * generated by the provided {@code IntSupplier}.  This is suitable for
   * generating constant streams, streams of random elements, etc.
   *
   * @param s the {@code IntSupplier} for generated elements
   * @return a new infinite sequential unordered {@code IntStream}
   */
  public static IntStream generate(IntSupplier s) {
    Objects.requireNonNull(s);
    return StreamSupport.intStream(
        new StreamSpliterators.InfiniteSupplyingSpliterator.OfInt(Long.MAX_VALUE, s), false);
  }

  /**
   * Returns a sequential ordered {@code IntStream} from {@code startInclusive}
   * (inclusive) to {@code endExclusive} (exclusive) by an incremental step of
   * {@code 1}.
   *
   * @param startInclusive the (inclusive) initial value
   * @param endExclusive the exclusive upper bound
   * @return a sequential {@code IntStream} for the range of {@code int} elements
   * @apiNote <p>An equivalent sequence of increasing values can be produced sequentially using a
   * {@code for} loop as follows:
   * <pre>{@code
   *     for (int i = startInclusive; i < endExclusive ; i++) { ... }
   * }</pre>
   */
  public static IntStream range(int startInclusive, int endExclusive) {
    if (startInclusive >= endExclusive) {
      return empty();
    } else {
      return StreamSupport.intStream(
          new Streams.RangeIntSpliterator(startInclusive, endExclusive, false), false);
    }
  }

  /**
   * Returns a sequential ordered {@code IntStream} from {@code startInclusive}
   * (inclusive) to {@code endInclusive} (inclusive) by an incremental step of
   * {@code 1}.
   *
   * @param startInclusive the (inclusive) initial value
   * @param endInclusive the inclusive upper bound
   * @return a sequential {@code IntStream} for the range of {@code int} elements
   * @apiNote <p>An equivalent sequence of increasing values can be produced sequentially using a
   * {@code for} loop as follows:
   * <pre>{@code
   *     for (int i = startInclusive; i <= endInclusive ; i++) { ... }
   * }</pre>
   */
  public static IntStream rangeClosed(int startInclusive, int endInclusive) {
    if (startInclusive > endInclusive) {
      return empty();
    } else {
      return StreamSupport.intStream(
          new Streams.RangeIntSpliterator(startInclusive, endInclusive, true), false);
    }
  }

  /**
   * Creates a lazily concatenated stream whose elements are all the
   * elements of the first stream followed by all the elements of the
   * second stream.  The resulting stream is ordered if both
   * of the input streams are ordered, and parallel if either of the input
   * streams is parallel.  When the resulting stream is closed, the close
   * handlers for both input streams are invoked.
   *
   * @param a the first stream
   * @param b the second stream
   * @return the concatenation of the two input streams
   * @implNote Use caution when constructing streams from repeated concatenation. Accessing an
   * element of a deeply concatenated stream can result in deep call chains, or even {@code
   * StackOverflowException}.
   */
  public static IntStream concat(IntStream a, IntStream b) {
    Objects.requireNonNull(a);
    Objects.requireNonNull(b);

    Spliterator.OfInt split = new Streams.ConcatSpliterator.OfInt(
        a.spliterator(), b.spliterator());
    IntStream stream = StreamSupport.intStream(split, a.isParallel() || b.isParallel());
    return stream.onClose(Streams.composedClose(a, b));
  }

  /**
   * A mutable builder for an {@code IntStream}.
   *
   * <p>A stream builder has a lifecycle, which starts in a building
   * phase, during which elements can be added, and then transitions to a built
   * phase, after which elements may not be added.  The built phase
   * begins when the {@link #build()} method is called, which creates an
   * ordered stream whose elements are the elements that were added to the
   * stream builder, in the order they were added.
   *
   * @see IntStream#builder()
   * @since 1.8
   */
  public interface Builder extends IntConsumer {

    /**
     * Adds an element to the stream being built.
     *
     * @throws IllegalStateException if the builder has already transitioned to the built state
     */
    @Override
    void accept(int t);

    /**
     * Adds an element to the stream being built.
     *
     * @param t the element to add
     * @return {@code this} builder
     * @throws IllegalStateException if the builder has already transitioned to the built state
     * @implSpec The default implementation behaves as if:
     * <pre>{@code
     *     accept(t)
     *     return this;
     * }</pre>
     */
    default Builder add(int t) {
      accept(t);
      return this;
    }

    /**
     * Builds the stream, transitioning this builder to the built state.
     * An {@code IllegalStateException} is thrown if there are further
     * attempts to operate on the builder after it has entered the built
     * state.
     *
     * @return the built stream
     * @throws IllegalStateException if the builder has already transitioned to the built state
     */
    IntStream build();
  }
}
